In the oil exploration and production industries, it is important to know the density and viscosity of reservoir fluids, such as crude oil or brine, at the pressure and temperature of the reservoir. These properties are used to determine the permeability and flow characteristics of the reservoir as well as the commercial value of the fluid in place. It is current practice to obtain a fluid sample downhole during the exploration phase of oilfield development and to determine the fluid's thermo-physical properties at the surface. Although the pressure and temperature of the fluid sample at the surface can be adjusted to those at reservoir conditions, there is considerable difficulty in obtaining a sample that closely resembles the downhole fluid chemical composition owing to the volatility of lighter hydrocarbons, solids deposition, and drilling fluid contamination. The cost of retrieving a downhole sample and the difficulty of handling samples at the surface under downhole pressure and temperature conditions are both very high. Making measurements downhole will accelerate the evaluation process and reduce total costs. Decisions concerning reservoir production and optimization activities are often based on analyses of extremely small fluid samples obtained downhole, by volume relatively less than 10−9 of the reserves within a typical reservoir. The composition of a reservoir fluid can and will change during the lifetime of a reserve and thus the fluid properties will change. More frequent fluid property measurements, taken throughout the exploration and production process, would be extremely useful.
Downhole conditions are far more extreme than typical fluid property sensors are capable of operating under. At this time, fluids in a majority of producing hydrocarbon reservoirs are at downhole temperatures between (50 and 175) degrees Celsius, at downhole pressures between (100 and 2,000) bar, have densities in the range (500 to 1300) kg m−3, and have viscosities on the order of (1 to 1000) mPa s. Oilfield equipment typically must pass rigorous shock and corrosion resistance standards, due to the difficult deployment environment and the possible presence of corrosive fluid constituents such as H2S and CO2. Reservoir fluids are often extremely complex and may contain chemical components ranging from asphaltenes and waxes to methane. In this environment, it is necessary to have a fluid sensor that can operate accurately in this complicated and harsh environment. No commercially available device exists today that would satisfy these requirements. Fluid property sensors are also utilized in many other industries, such as the water industry, the chemical processing industry, and the food processing industry. Improved fluid property sensors could offer substantial benefits to these types of industries as well.